Ethylene homopolymers and copolymers are a well-known class of olefin polymers from which various plastic products are produced. Such products include hot melt adhesives. The polymers used to make such adhesives can be prepared from ethylene, optionally with one or more copolymerizable monomers. One process used to produce ethylene homopolymers and copolymers involves use of a coordination catalyst, such as a Ziegler-Natta catalyst, under low pressures. Conventional Ziegler-Natta catalysts are typically composed of many types of catalytic species, each having different metal oxidation states and different coordination environments with ligands. Examples of such heterogeneous systems are known and include metal halides activated by an organometallic co-catalyst, such as titanium chloride supported on magnesium chloride, activated with trialkylaluminum compounds. Because these systems contain more than one catalytic species, they possess polymerization sites with different activities and varying abilities to incorporate comonomer into a polymer chain. The consequence of such multi-site chemistry is a product with poor control of the polymer chain architecture, when compared to a neighboring chain. Moreover, differences in the individual catalyst site produce polymers of high molecular weight at some sites and low molecular weight at others, resulting in a polymer with a heterogeneous composition. The molecular weight distribution (as indicated by Mw/Mn, also referred to as polydispersity index or “PDI” or “MWD”) of such polymers can be fairly broad. For some combinations of heterogeneity and broad MWD, the mechanical and other properties of the polymers are sometimes less desirable in certain applications than in others.
Another catalyst technology useful in the polymerization of olefins is based on the chemistry of single-site homogeneous catalysts, including metallocenes which are organometallic compounds containing one or more cyclopentadienyl ligands attached to a metal, such as hafnium, titanium, vanadium, or zirconium. A co-catalyst, such as oligomeric methylaluminoxane (also called methylalumoxane), is often used to promote the catalytic activity of the catalyst.
The uniqueness of single site catalysts, including metallocenes, resides in part in the steric and electronic equivalence of each active catalyst site. Specifically, these catalysts are characterized as having a single, stable chemical site rather than a mixture of sites as discussed above for conventional Ziegler-Natta catalysts. The resulting system is composed of catalytic species which have a singular activity and selectivity. Polymers produced by such catalysts are often referred to as homogeneous- or single site-resins in the art.
A consequence of such singular reactivity is that by variation in the metal component and/or the ligands and ligand substituents of the transition metal complex component of the single site catalyst, a myriad of polymer products may be tailored. These include oligomers and polymers with molecular weights (Mn) ranging from about 200 to greater than 1,000,000. In addition, by varying the metal component and/or the ligands and ligand substituents of the single site catalyst, it is also possible in ethylene alpha olefin interpolymerizations to vary the comonomer reactivity of the catalyst, such that very different levels of comonomer are incorporated at a given comonomer concentration. Thus it is also possible to tailor the density of the product from products with high comonomer incorporation (resulting in densities lower than 0.900 g/cm3), through to products with almost no comonomer incorporation (resulting in densities greater than 0.950 g/cm3), both at the same comonomer concentration in the reactor.
One method of utilizing this variation in single site catalyst reactivity is to employ two or more such catalysts in conjunction with a multiple reactor configuration, to produce so-called in reactor resin blends which are a combination of products made by each catalyst In this case, there exists the ability to: i) control the polymerization conditions in each reactor independently, ii) control the contribution of each reactor product to the final polymer composition (the so called reactor split ratio) and iii) supply each reactor with a single-site catalyst, allows such a process to produce a wide range of polymeric products that are combinations of each reactor product. The ability to produce such in-reactor blends as opposed to post reactor blending of separately prepared components has definite process, economic and product flexibility advantages in applications calling for a product which cannot be made in a single reactor single catalyst or dual reactor single catalyst configuration.
In addition, the mutual compatibility of single site catalyst mixtures (as opposed to a mixture of a single site and traditional Ziegler catalyst) also allows for the possibility of producing a broad range of in-reactor blend products in a single reactor, even under the same polymerization conditions by introducing single site catalysts of differing comonomer reactivity and/or termination kinetics into the reactor, and varying their relative amounts to yield the desired final polymer properties. In this mode, in-reactor blends may also be prepared which again are otherwise unavailable except by post reactor blending of separately prepared components.
There a number of examples of both types of products and processes in the prior art. For instance, U.S. Pat. No. 5,530,065 (Farley et al.) discloses heat sealed articles and heat sealable films comprising a polymer blend of a first polymer having a narrow molecular weight distribution and composition distribution and a second polymer having a broad molecular weight distribution and composition distribution.
U.S. Pat. Nos. 5,382,630 and 5,382,631 (Stehling et al.) discloses linear ethylene interpolymer blends with improved properties made from components having a narrow molecular weight distribution (Mw/Mn≦3) and a narrow composition distribution (CDBI>50%).
U.S. Pat. No. 6,545,088 B1 (Kolthammer et al.) discloses a process for polymerizing ethylene, an alpha-olefin and optionally a diene catalyzed by a metallocene catalyst in either a single or multiple reactors.
U.S. Pat. No. 6,566,446 B1 (Kolthammer et al.) discloses a process comprising interpolymerizing a first homogeneous ethylene/alpha-olefin interpolymer and at least one second homogeneous ethylene/alpha-olefin interpolymer using at least two constrained geometry catalysts. The catalysts have different reactivities such that the first interpolymer has a narrow molecular weight distribution and a very high comonomer content and relatively high molecular weight, and the second ethylene/alpha olefin interpolymer has a narrow molecular weight distribution and a low comonomer content and a molecular weight lower than that of the first interpolymer. The interpolymers can be polymerized in a single reactor or separate reactors operated in parallel of series.
WO 97/48735 (Canich et al.) discloses a mixed transition metal olefin polymerization catalyst system comprising one late transition metal catalyst and at least one different catalyst system selected from the group consisting of late transition metal catalyst systems, transition metal metallocene catalyst systems or Ziegler-Natta catalyst systems.
U.S. Pat. No. 4,939,217 (Stricklen) discloses a process for producing a polyolefin having a multimodal molecular weight distribution wherein the polymerization is conducted in the presence of hydrogen and a catalyst system containing alumoxane and at least two different metallocenes each having different olefin polymerization termination rate constants.
U.S. Pat. No. 4,937,299 (Ewen et al.) discloses polyolefin reactor blends obtained by polymerization of ethylene and higher alpha-olefins in the presence of a catalyst system comprising two or more metallocenes and alumoxane.
WO 02/074816A2 (deGroot et al.) discloses a polymer composition (and process for making) which comprises: (a) a high molecular weight, branched component; and (b) a low molecular weight, branched component.
WO 02/074817A2 (Stevens et al.) discloses a polymerization process which comprises contacting one or more olefinic comonomers in the presence of at least a high molecular weight catalyst and at least a low molecular weight catalyst in a single reactor; and effectuating the polymerization of the olefinic comonomers in the reactor to obtain an olefin polymer, whereby both catalysts have the ability to incorporate a substantially similar amount of comonomers in the olefin polymer.
Such flexibility in polymer preparation is highly desirable in certain applications, which call for a special and unique combination of polymer properties. One such example is a polymer formulation employed in hot melt adhesive (“HMA”) formulations. Most hot melt adhesives are three component mixtures of a polymeric resin, a wax, and a tackifying agent. Although each component is generally present in roughly equal proportions in an HMA formulation, their relative ratio is often “fine tuned” for a particular application's need. Typically, the polymer component provides the strength to the adhesive bond, while the wax reduces the overall viscosity of the system simplifying application of the adhesive to the substrate to be bonded.
The polymeric resin of an HMA can be ethylene homopolymers and interpolymers of a selected molecular weight and density. Such interpolymers can be a single polymer or a blend composition. For instance, U.S. Pat. No. 5,530,054, issued Jun. 25, 1996 to Tse et al., claims a hot melt adhesive composition consisting essentially of: (a) 30 percent to 70 percent by weight of a copolymer of ethylene and about 6 percent to about 30 percent by weight of a C3 to C20 α-olefin produced in the presence of a catalyst composition comprising a metallocene and an alumoxane and having an MW of from about 20,000 to about 100,000; and (b) a hydrocarbon tackifier which is selected from a recited list.
U.S. Pat. No. 5,548,014, issued Aug. 20, 1996 to Tse et al., claims a hot melt adhesive composition comprising a blend of ethylene/alpha-olefin copolymers wherein the first copolymer has a MW from about 20,000 to about 39,000 and the second copolymer has a MW from about 40,000 to about 100,000. Each of the hot melt adhesives exemplified comprises a blend of copolymers, with at least one of the copolymers having a polydispersity greater than 2.5. Furthermore, the lowest density copolymer exemplified has a specific gravity of 0.894 g/cm3.
However, it would be highly advantageous in such HMA applications to have access to a synthetic polymer with properties such that it can substitute for both the wax and polymer components of a hot melt adhesive formulation.
It would also be highly advantageous to have a process for preparing such polymer composition comprising a minimum of mixing steps, thus minimizing the cost and variability of the formulation.
It would also be highly advantageous to have a polymer composition for use in an HMA formulation, and a process for its preparation which negates the requirement of incorporating expensive petroleum waxes into hot melt adhesive formulations that are primarily imported and or derived from imported oil based feedstocks.
Finally, it would also be highly advantageous to have access to a synthetic polymer: i) with properties such that it can substitute for both the wax and polymer components of a hot melt adhesive formulation; ii) which can be prepared by a process comprising a minimum of mixing steps, thus minimizing the cost and variability of the formulation; iii) which when incorporated into a hot melt adhesive formulation, negates the need for expensive petroleum waxes (primarily imported and or derived from imported oil based feedstocks) in hot melt adhesive formulations; and iv) which when incorporated into HMA formulations, said formulations are able to exhibit the strength and adhesion characteristics of commercial HMAs, while also exhibiting improved thermal and oxidative stability.